Abnormal formation of sarcoplasmic reticulum networks and triads during early development of skeletal muscle cells in mitsugumin29-deficient mice

The Sarcoplasmic Reticulum of Skeletal Muscle Cells: A Labyrinth of Membrane Contact Sites

The sarcoplasmic reticulum of skeletal muscle cells is a highly ordered structure consisting of an intricate network of tubules and cisternae specialized for regulating Ca2+ homeostasis in the context of muscle contraction. The sarcoplasmic reticulum contains several proteins, some of which support Ca2+ storage and release, while others regulate the formation and maintenance of this highly convoluted organelle and mediate the interaction with other components of the muscle fiber. In this review, some of the main issues concerning the biology of the sarcoplasmic reticulum will be described and discussed; particular attention will be addressed to the structure and function of the two domains of the sarcoplasmic reticulum supporting the excitation-contraction coupling and Ca2+-uptake mechanisms.

Structure-Function Relationships and Modifications of Cardiac Sarcoplasmic Reticulum Ca2+-Transport

Sarcoplasmic reticulum (SR) is a specialized tubular network, which not only maintains the intracellular concentration of Ca2+ at a low level but is also known to release and accumulate Ca2+ for the occurrence of cardiac contraction and relaxation, respectively. This subcellular organelle is composed of several phospholipids and different Ca2+-cycling, Ca2+-binding and regulatory proteins, which work in a coordinated manner to determine its function in cardiomyocytes. Some of the major proteins in the cardiac SR membrane include Ca2+-pump ATPase (SERCA2), Ca2+-release protein (ryanodine receptor), calsequestrin (Ca2+-binding protein) and phospholamban (regulatory protein). The phosphorylation of SR Ca2+-cycling proteins by protein kinase A or Ca2+-calmodulin kinase (directly or indirectly) has been demonstrated to augment SR Ca2+-release and Ca2+-uptake activities and promote cardiac contraction and relaxation functions. The activation of phospholipases and proteases as well as changes in different gene expressions under different pathological conditions have been shown to alter the SR composition and produce Ca2+-handling abnormalities in cardiomyocytes for the development of cardiac dysfunction. The post-translational modifications of SR Ca2+ cycling proteins by processes such as oxidation, nitrosylation, glycosylation, lipidation, acetylation, sumoylation, and O GlcNacylation have also been reported to affect the SR Ca2+ release and uptake activities as well as cardiac contractile activity. The SR function in the heart is also influenced in association with changes in cardiac performance by several hormones including thyroid hormones and adiponectin as well as by exercise-training. On the basis of such observations, it is suggested that both Ca2+-cycling and regulatory proteins in the SR membranes are intimately involved in determining the status of cardiac function and are thus excellent targets for drug development for the treatment of heart disease.

Loss of dysferlin or myoferlin results in differential defects in excitation-contraction coupling in mouse skeletal muscle

Muscular dystrophies are disorders characterized by progressive muscle loss and weakness that are both genotypically and phenotypically heterogenous. Progression of muscle disease arises from impaired regeneration, plasma membrane instability, defective membrane repair, and calcium mishandling. The ferlin protein family, including dysferlin and myoferlin, are calcium-binding, membrane-associated proteins that regulate membrane fusion, trafficking, and tubule formation. Mice lacking dysferlin (Dysf), myoferlin (Myof), and both dysferlin and myoferlin (Fer) on an isogenic inbred 129 background were previously demonstrated that loss of both dysferlin and myoferlin resulted in more severe muscle disease than loss of either gene alone. Furthermore, Fer mice had disordered triad organization with visibly malformed transverse tubules and sarcoplasmic reticulum, suggesting distinct roles of dysferlin and myoferlin. To assess the physiological role of disorganized triads, we now assessed excitation contraction (EC) coupling in these models. We identified differential abnormalities in EC coupling and ryanodine receptor disruption in flexor digitorum brevis myofibers isolated from ferlin mutant mice. We found that loss of dysferlin alone preserved sensitivity for EC coupling and was associated with larger ryanodine receptor clusters compared to wildtype myofibers. Loss of myoferlin alone or together with a loss of dysferlin reduced sensitivity for EC coupling, and produced disorganized and smaller ryanodine receptor cluster size compared to wildtype myofibers. These data reveal impaired EC coupling in Myof and Fer myofibers and slightly potentiated EC coupling in Dysf myofibers. Despite high homology, dysferlin and myoferlin have differential roles in regulating sarcotubular formation and maintenance resulting in unique impairments in calcium handling properties.

With the greatest care, stromal interaction molecule (STIM) proteins verify what skeletal muscle is doing

Skeletal muscle contracts or relaxes to maintain the body position and locomotion. For the contraction and relaxation of skeletal muscle, Ca²⁺ in the cytosol of skeletal muscle fibers acts as a switch to turn on and off a series of contractile proteins. The cytosolic Ca²⁺ level in skeletal muscle fibers is governed mainly by movements of Ca²⁺ between the cytosol and the sarcoplasmic reticulum (SR). Store-operated Ca²⁺ entry (SOCE), a Ca²⁺ entryway from the extracellular space to the cytosol, has gained a significant amount of attention from muscle physiologists. Orai1 and stromal interaction molecule 1 (STIM1) are the main protein identities of SOCE. This mini-review focuses on the roles of STIM proteins and SOCE in the physiological and pathophysiological functions of skeletal muscle and in their correlations with recently identified proteins, as well as historical proteins that are known to mediate skeletal muscle function.

A focus on extracellular Ca2+ entry into skeletal muscle

The main task of skeletal muscle is contraction and relaxation for body movement and posture maintenance. During contraction and relaxation, Ca²⁺ in the cytosol has a critical role in activating and deactivating a series of contractile proteins. In skeletal muscle, the cytosolic Ca²⁺ level is mainly determined by Ca²⁺ movements between the cytosol and the sarcoplasmic reticulum. The importance of Ca²⁺ entry from extracellular spaces to the cytosol has gained significant attention over the past decade. Store-operated Ca²⁺ entry with a low amplitude and relatively slow kinetics is a main extracellular Ca²⁺ entryway into skeletal muscle. Herein, recent studies on extracellular Ca²⁺ entry into skeletal muscle are reviewed along with descriptions of the proteins that are related to extracellular Ca²⁺ entry and their influences on skeletal muscle function and disease.

Mitsugumin 29 regulates t-tubule architecture in the failing heart

Transverse tubules (t-tubules) are uniquely-adapted membrane invaginations in cardiac myocytes that facilitate the synchronous release of Ca²⁺ from internal stores and subsequent myofilament contraction, although these structures become disorganized and rarefied in heart failure. We previously observed that mitsugumin 29 (Mg29), an important t-tubule organizing protein in skeletal muscle, was induced in the mouse heart for the first time during dilated cardiomyopathy with heart failure. Here we generated cardiac-specific transgenic mice expressing Mg29 to model this observed induction in the failing heart. Interestingly, expression of Mg29 in the hearts of Csrp3 null mice (encoding muscle LIM protein, MLP) partially restored t-tubule structure and preserved cardiac function as measured by invasive hemodynamics, without altering Ca²⁺ spark frequency. Conversely, gene-deleted mice lacking both Mg29 and MLP protein showed a further reduction in t-tubule organization and accelerated heart failure. Thus, induction of Mg29 in the failing heart is a compensatory response that directly counteracts the well-characterized loss of t-tubule complexity and reduced expression of anchoring proteins such as junctophilin-2 (Jph2) that normally occur in this disease. Moreover, preservation of t-tubule structure by Mg29 induction significantly increases the function of the failing heart.

Interaction between mitsugumin 29 and TRPC3 participates in regulating Ca(2+) transients in skeletal muscle

Mitsugumin 29 (MG29) is related to the fatigue and aging processes of skeletal muscle. To examine the roles of MG29 in conjunction with its binding protein, the canonical-type transient receptor potential cation channel 3 (TRPC3), in skeletal muscle, the binding region of MG29 to TRPC3 was studied along with the functional relevance of the binding in mouse primary skeletal myotubes using co-immunoprecipitation assays and Ca(2+) imaging experiments. The N-terminus and the I-II loop of MG29 constitute the binding region for TRPC3. The myotubes that expressed the MG29 mutant missing the entire TRPC3-binding region showed a disrupted binding between endogenous MG29 and TRPC3 and a reduction in Ca(2+) transients in response to membrane depolarization without affecting ryanodine receptor 1 activity, the resting cytosolic Ca(2+) level, and the amount of releasable Ca(2+) from the sarcoplasmic reticulum. Among the proteins mediating Ca(2+) movements in skeletal muscle, TRPC4 expression was significantly decreased by the MG29 mutant. Therefore, MG29 could be a new factor for regulating Ca(2+) transients during skeletal muscle contraction possibly via a correlation with TRPC3 and TRPC4.

Ca2+-dependent regulations and signaling in skeletal muscle: from electro-mechanical coupling to adaptation

Calcium (Ca2+) plays a pivotal role in almost all cellular processes and ensures the functionality of an organism. In skeletal muscle fibers, Ca(2+) is critically involved in the innervation of skeletal muscle fibers that results in the exertion of an action potential along the muscle fiber membrane, the prerequisite for skeletal muscle contraction. Furthermore and among others, Ca(2+) regulates also intracellular processes, such as myosin-actin cross bridging, protein synthesis, protein degradation and fiber type shifting by the control of Ca(2+)-sensitive proteases and transcription factors, as well as mitochondrial adaptations, plasticity and respiration. These data highlight the overwhelming significance of Ca(2+) ions for the integrity of skeletal muscle tissue. In this review, we address the major functions of Ca(2+) ions in adult muscle but also highlight recent findings of critical Ca(2+)-dependent mechanisms essential for skeletal muscle-regulation and maintenance.

Store-operated Ca2+ entry in muscle physiology and diseases

Ca(2+) release from intracellular stores and influx from extracellular reservoir regulate a wide range of physiological functions including muscle contraction and rhythmic heartbeat. One of the most ubiquitous pathways involved in controlled Ca(2+) influx into cells is store-operated Ca(2+) entry (SOCE), which is activated by the reduction of Ca(2+) concentration in the lumen of endoplasmic or sarcoplasmic reticulum (ER/SR). Although SOCE is pronounced in non-excitable cells, accumulating evidences highlight its presence and important roles in skeletal muscle and heart. Recent discovery of STIM proteins as ER/SR Ca(2+) sensors and Orai proteins as Ca(2+) channel pore forming unit expedited the mechanistic understanding of this pathway. This review focuses on current advances of SOCE components, regulation and physiologic and pathophysiologic roles in muscles. The specific property and the dysfunction of this pathway in muscle diseases, and new directions for future research in this rapidly growing field are discussed.

Trimeric intracellular cation channels and sarcoplasmic/endoplasmic reticulum calcium homeostasis

Trimeric intracellular cation channels (TRIC) represents a novel class of trimeric intracellular cation channels. Two TRIC isoforms have been identified in both the human and the mouse genomes: TRIC-A, a subtype predominantly expressed in the sarcoplasmic reticulum (SR) of muscle cells, and TRIC-B, a ubiquitous subtype expressed in the endoplasmic reticulum (ER) of all tissues. Genetic ablation of either TRIC-A or TRIC-B leads to compromised K(+) permeation and Ca(2+) release across the SR/ER membrane, supporting the hypothesis that TRIC channels provide a counter balancing K(+) flux that reduces SR/ER membrane depolarization for maintenance of the electrochemical gradient that drives SR/ER Ca(2+) release. TRIC-A and TRIC-B seem to have differential functions in Ca(2+) signaling in excitable and nonexcitable cells. Tric-a(-/-) mice display defective Ca(2+) sparks and spontaneous transient outward currents in arterial smooth muscle and develop hypertension, in addition to skeletal muscle dysfunction. Knockout of TRIC-B results in abnormal IP3 receptor-mediated Ca(2+) release in airway epithelial cells, respiratory defects, and neonatal lethality. Double knockout mice lacking both TRIC-A and TRIC-B show embryonic lethality as a result of cardiac arrest. Such an aggravated lethality indicates that TRIC-A and TRIC-B share complementary physiological functions in Ca(2+) signaling in embryonic cardiomyocytes. Tric-a(-/-) and Tric-b(+/-) mice are viable and susceptible to stress-induced heart failure. Recent evidence suggests that TRIC-A directly modulates the function of the cardiac ryanodine receptor 2 Ca(2+) release channel, which in turn controls store-overload-induced Ca(2+) release from the SR. Thus, the TRIC channels, in addition to providing a countercurrent for SR/ER Ca(2+) release, may also function as accessory proteins that directly modulate the ryanodine receptor/IP3 receptor channel functions.

Junctophilin-2 is necessary for T-tubule maturation during mouse heart development

AIMS

Transverse tubules (TTs) provide the basic subcellular structures that facilitate excitation-contraction (EC) coupling, the essential process that underlies normal cardiac contractility. Previous studies have shown that TTs develop within the first few weeks of life in mammals but the molecular determinants of this development have remained elusive. This study aims to elucidate the role of junctophilin-2 (JPH2), a junctional membrane complex protein, in the maturation of TTs in cardiomyocytes.

METHODS AND RESULTS

Using a novel cardiac-specific short-hairpin-RNA-mediated JPH2 knockdown mouse model (Mus musculus; αMHC-shJPH2), we assessed the effects of the loss of JPH2 on the maturation of the ventricular TT structure. Between embryonic day (E) 10.5 and postnatal day (P) 10, JPH2 mRNA and protein levels were reduced by >70% in αMHC-shJPH2 mice. At P8 and P10, knockdown of JPH2 significantly inhibited the maturation of TTs, while expression levels of other genes implicated in TT development remained mostly unchanged. At the same time, intracellular Ca(2+) handling was disrupted in ventricular myocytes from αMHC- shJPH2 mice, which developed heart failure by P10 marked by reduced ejection fraction, ventricular dilation, and premature death. In contrast, JPH2 transgenic mice exhibited accelerated TT maturation by P8.

CONCLUSION

Our findings suggest that JPH2 is necessary for TT maturation during postnatal cardiac development in mice. In particular, JPH2 may be critical in anchoring the invaginating sarcolemma to the sarcoplasmic reticulum, thereby enabling the maturation of the TT network.

T-tubule biogenesis and triad formation in skeletal muscle and implication in human diseases

In skeletal muscle, the excitation-contraction (EC) coupling machinery mediates the translation of the action potential transmitted by the nerve into intracellular calcium release and muscle contraction. EC coupling requires a highly specialized membranous structure, the triad, composed of a central T-tubule surrounded by two terminal cisternae from the sarcoplasmic reticulum. While several proteins located on these structures have been identified, mechanisms governing T-tubule biogenesis and triad formation remain largely unknown. Here, we provide a description of triad structure and plasticity and review the role of proteins that have been linked to T-tubule biogenesis and triad formation and/or maintenance specifically in skeletal muscle: caveolin 3, amphiphysin 2, dysferlin, mitsugumins, junctophilins, myotubularin, ryanodine receptor, and dihydhropyridine Receptor. The importance of these proteins in triad biogenesis and subsequently in muscle contraction is sustained by studies on animal models and by the direct implication of most of these proteins in human myopathies.

Cytoskeletal tropomyosin Tm5NM1 is required for normal excitation-contraction coupling in skeletal muscle

The functional diversity of the actin microfilaments relies in part on the actin binding protein tropomyosin (Tm). The muscle-specific Tms regulate actin-myosin interactions and hence contraction. However, there is less known about the roles of the numerous cytoskeletal isoforms. We have shown previously that a cytoskeletal Tm, Tm5NM1, defines a Z-line adjacent cytoskeleton in skeletal muscle. Recently, we identified a second cytoskeletal Tm in this region, Tm4. Here we show that Tm4 and Tm5NM1 define separate actin filaments; the former associated with the terminal sarcoplasmic reticulum (SR) and other tubulovesicular structures. In skeletal muscles of Tm5NM1 knockout (KO) mice, Tm4 localization was unchanged, demonstrating the specificity of the membrane association. Tm5NM1 KO muscles exhibit potentiation of T-system depolarization and decreased force rundown with repeated T-tubule depolarizations consistent with altered T-tubule function. These results indicate that a Tm5NM1-defined actin cytoskeleton is required for the normal excitation-contraction coupling in skeletal muscle.

Altered Ca2+ sparks in aging skeletal and cardiac muscle

Ca2+ sparks are the fundamental units that comprise Ca2+-induced Ca2+ release (CICR) in striated muscle cells. In cardiac muscle, spontaneous Ca2+ sparks underlie the rhythmic CICR activity during heart contraction. In skeletal muscle, Ca2+ sparks remain quiescent during the resting state and are activated in a plastic fashion to accommodate various levels of stress. With aging, the plastic Ca2+ spark signal becomes static in skeletal muscle, whereas loss of CICR control leads to leaky Ca2+ spark activity in aged cardiomyocytes. Ca2+ spark responses reflect the integrated function of the intracellular Ca2+ regulatory machinery centered around the triad or dyad junctional complexes of striated muscles, which harbor the principal molecular players of excitation-contraction coupling. This review highlights the contribution of age-related modification of the Ca2+ release machinery and the effect of membrane structure and membrane cross-talk on the altered Ca2+ spark signaling during aging of striated muscles.

The sarcoplasmic reticulum: an organized patchwork of specialized domains

The sarcoplasmic reticulum (SR) of skeletal muscle cells is a convoluted structure composed of a variety of tubules and cisternae, which share a continuous lumen delimited by a single continuous membrane, branching to form a network that surrounds each myofibril. In this network, some specific domains basically represented by the longitudinal SR and the junctional SR can be distinguished. These domains are mainly dedicated to Ca(2+) homeostasis in relation to regulation of muscle contraction, with the longitudinal SR representing the sites of Ca(2+) uptake and storage and the junctional SR representing the sites of Ca(2+) release. To perform its functions, the SR takes contact with other cellular elements, the sarcolemma, the contractile apparatus and the mitochondria, giving rise to a number of interactions, most of which are still to be defined at the molecular level. This review will describe some of the most recent advancements in understanding the organization of this complex network and its specific domains. Furthermore, we shall address initial evidence on how SR proteins are retained at distinct SR domains.

Immuno-proteomic approach to excitation--contraction coupling in skeletal and cardiac muscle: molecular insights revealed by the mitsugumins

A comprehensive understanding of excitation-contraction (E-C) coupling in skeletal and cardiac muscle requires that all the major components of the Ca(2+) release machinery be resolved. We utilized a unique immuno-proteomic approach to generate a monoclonal antibody library that targets proteins localized to the skeletal muscle triad junction, which provides a structural context to allow efficient E-C coupling. Screening of this library has identified several mitsugumins (MG); proteins that can be localized to the triad junction in mammalian skeletal muscle. Many of these proteins, including MG29 and junctophilin, are important components in maintaining the structural integrity of the triad junction. Other triad proteins, such as calumin, play a more direct role in regulation of muscle Ca(2+) homeostasis. We have recently identified a family of trimeric intracellular cation-selective (TRIC) channels that allow for K(+) movement into the endoplasmic or sarcoplasmic reticulum to counter a portion of the transient negative charge produced by Ca(2+) release into the cytosol. Further study of TRIC channel function and other novel mitsugumins will increase our understanding of E-C coupling and Ca(2+) homoeostasis in muscle physiology and pathophysiology.

Deficiency of triad formation in developing skeletal muscle cells lacking junctophilin type 1

Junctophilins (JP-1, JP-2, and JP-3) are transmembrane proteins expressed in the junctional membrane complexes in excitable cells. Both JP-1 and JP-2 are co-expressed in the triads of skeletal muscle, but only JP-2 is expressed in cardiac muscle. We analyzed the roles played by JP-1 and JP-2 in triad formation in skeletal muscle by comparing developing skeletal muscles in wild-type and JP-1-knockout (KO) mice (both before and after birth). In the skeletal muscles of embryos, most of the couplings between sarcoplasmic reticulum (SR) and transverse tubule (T-tubule) were diads, with triads being very scarce. The number of triads increased markedly after birth in wild-type mice. However, there was no increase in the number of triads in the neonates of JP-1-KO mice, and they died within 1 day after birth. JP-2 expression was constant before and after birth, while expression of JP-1 increased with birth. Quantitative and morphological differences were not seen between wild-type and JP-1-KO mice in the formation of diads in the period just before the JP-1-KO mice died. The SR swelled and developed large vacuoles in skeletal muscle cells just before the JP-1-KO mice died. The present results strongly suggest that JP-1 and JP-2 play important roles in the formation of triads and diads, respectively, during the development of skeletal muscle in mouse.